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A dimming of the Sun to match conditions in the 'Little Ice Age' of the 17th century would only slightly slow global warming, according to new research.

The study, which appears in the journal Geophysical Research Letters, reveals a weakening of solar activity in recent years, linked to fewer sunspots, would cut at most 0.3°C from a projected rise in temperatures by 2100 if it becomes a long-lasting 'Grand Minimum' of brightness.

World temperatures are likely to rise by between 3.7°C and 4.5°C by 2100 if greenhouse gas emissions keep building up - far more than the impact of known shifts in solar output, the study shows.

The Sun has gone through four Grand Minima since the 13th century, including the Maunder Minimum from 1645-1715 that overlapped with the Little Ice Age. The Thames River froze in London, for instance, during a 'Great Frost' of 1683-84.

World temperatures have risen 0.7°C since the Industrial Revolution led to increasing use of fossil fuels that release greenhouse gases when burnt, according to the UN panel of climate scientists.

Dim Sun

"Current temperature data also confirm that the effect of low solar activity on the climate is very small," says Professor Stefan Rahmstorf, also of the Potsdam Institute.

Despite a deep winter chill in parts of Europe and North America, January 2010 was the equal second warmest January worldwide since records began in the 19th century, according to NASA data. The warmest January was in 2007.

Feulner says those who doubted that human activity was to blame for global warming had wrongly suggested that a prolonged solar slowdown "might rescue us from global warming."

"We have experienced a low and long solar minimum on its current 11-year cycle. Some solar scientists have suggested that it might indicate the start of a type of Maunder Minimum," says Feulner said.

Consistent finding

Julie Arblaster, a climate researcher at the Australian Bureau of Meteorology, says it's the first time future projections have included the possibility that the Sun might go into an 'extended quiet period'.

"It's important that we try to understand how that would interact with greenhouse gases increasing," she says.

"It's definitely consistent with our study of the [11-year] solar cycle, which showed about a 0.1 degree change in temperature from a 0.1% change in the Sun - they're getting three times the effect for three times the amount of forcing," she says.

According to Arblaster a reduction in solar irradiance of 0.25%, used in the study, is "on the extreme end of what we would expect for the next century."

"This shows that any changes in the Sun, even large changes, will only have a small impact in offsetting that warming."